This invention relates to a network system. The recent prevalence of the Internet is enhancing the demand for higher-speed network communications. To meet this demand for higher-speed communications, ADSL (Asymmetric Digital Subscriber Line) or PON (Passive Optical Network) is growing. The PON includes B-PON (Broadband PON), E-PON (Ethernet PON), and G-PON (Gigabit-capable PON).
The PON is a network system which connects an accommodation station (OLT: Optical Line Terminal) installed in a vendor's station and network units (ONUs: Optical Network Units) installed in user premises; in the PON, the signal in an optical fiber connected to the OLT is split into a plurality of fibers with an optical splitter; the plurality of fibers are connected to the ONUs one by one. When a network is configured with the PON, low cost fiber installation and high-speed communications by employment of optical transmission are attained. For these reasons, the PON is spreading all over the world.
Among the techniques utilizing the PON, TDM-PON (Time Division Multiplexing Passive Optical Network) is widely employed, which uses optical signals having different wavelengths in downstream transmission from the OLT to ONUs and upstream transmission from the ONUs to the OLT and further applies time-division multiplexing to the signals depending on the ONU. This TDM-PON is employed in B-PON, E-PON, G-PON, 10G-EPON, and XG-PON.
In the TDM-PON, the OLT controls the timing of sending optical signals from ONUs to prevent a conflict among the optical signals from the ONUs in upstream transmission. Specifically, the OLT sends each ONU a control frame to specify a permitted transmission period. Each ONU sends an upstream control signal and upstream data during the period specified by the received control frame.
Other than the PON, the point-to-point type for communicating one to one is widely used for an optical access network. As a standard for the physical layer and the MAC layer in the point-to-point type network, 1000BASE-LX may be employed.
Traditionally, optical access networks of different standards have been used for different accommodated services and different communication carriers, so that optical fiber networks specific to the individual optical access methods have been constructed. For this reason, the use of a plurality of optical access methods have elevated the cost for apparatuses, the cost to construct optical fiber networks, and the cost for their maintenance and management.
Accordingly, desired is an optical access network that accommodates optical signals generated by various optical access methods into the same optical fiber network by wavelength-division multiplexing. In order to efficiently use multiple wavelength patterns as resources, it is desirable that the optical access network should not to fix the association relations between wavelengths and optical access methods but flexibly configure them depending on the optical access.
Because of a massive number of ONUs, manual configuration of wavelengths in ONUs complicates management; furthermore, some mistake in the configuration might cause malfunctions. Accordingly, demanded for the future are optical access networks that attain co-existence of various optical access methods, flexible configuration of access methods and wavelengths, and automatic wavelength configuration for individual ONUs.
There have been proposed wavelength-division multiplexing optical access systems that can flexibly configure the connection of OSUs and ONUs in order to accommodate various services (for example, refer to JP 2006-081014 A). JP 2006-081014 A discloses a technique that an OSU in an OLT superimposes a frequency tone on downstream signal light in transmission and each ONU sets a wavelength based on the frequency tone.